1. Field of the Invention
This invention relates generally to systems for non-destructively examining objects using penetrative radiation and, more particularly, relates to an improved method and apparatus for measuring the intensity of X-radiation emerging from the subject of a clinical examination.
2. Description of the Prior Art
A conventional radiograph is a two-dimentional shadow image of a three-dimensional subject. The depth dimension is not apparent since all interior structures of the subject appear to be in a single plane. As a consequence in some circumstances, a conventional radiograph may fail necessary detail concerning relative spatial locations of interior structures, is difficult to interpret, and may not reveal the existence of a condition of interest in the subject.
Tomographic procedures have been developed to fulfill some objectives which are unobtainable by conventional radiographical procedures. In tomography, an image of the subject's internal structure, located in substantially only a single cross-sectional plane of interest extending through a subject, is produced by sequentially directing X-rays through the subject from a plurality of directions. The resulting image reveals relative spatial relationships of internal structures of the subject in substantially only the plane of interest.
Early tomographic systems utilized a radiation detector whose movement was coordinated with movement of a radiation source which directed a radiation beam to the detector. The source-detector pair simultaneously moved about an axis passing through the subject, and the system produced a cross-sectional image of the subject substantially in a plane of interest containing the axis about which the source-detector pair moved. The detector-source motion of this scanning technique resulted in substantially greater changes in the spatial relationship between the radiation beam and the subject's internal structure outside the plane of interest than between the beam and the structure within that plane. These greater changes blurred images of the structures out of the plane of interest and had little or no effect on images of structure in the plane of interest. In this manner our image of the structures in substantially only the plane of interest was produced.
Other tomographic procedures have been proposed which develop a cross-sectional image of the subject taken in a plane which includes the axis of the X-ray beam. Tomography which produces such images is known as transaxial tomography. This type of tomography produces an image, or representation, of a transverse section through the subject being examined, by generating a plurality of discrete portions of information, each portion representing a part of such an image, and "reconstructing" the image from the represented parts.
Reconstruction tomography scanning has evolved into two general types of systems. In one such system radiation is emitted as the source-detector pair are translated linearly with the beam axes in the plane of interest of the subject to be examined. A number of such translations or scans are completed during each examination of a plane of interest, with the orientation of the of linear translation being changed from one scan to another. Each scan, or single linear translation, is divided into individual scan segments. The radiation passing through the subject during each scan segment constitutes, in effect, a single beam passing through the subject along a narrow path in the plane of interest. The integral detected intensity of the beam passing through the subject during each scan segment is recorded for computing X-ray transmission (or X-ray absorption) characteristics of the patient. These characteristics are appropriately processed to provide a reconstructed image of the internal structure of the subject in the plane of interest.
In another type of transaxial reconstruction tomographic scanning system, a radiation source-detector pair orbits about the subject in a circular path with the axis of a beam in a plane of interest. After each orbit the source-detector pair is incrementally pivoted about an axis passing through the source and normal to the plane of interest. Another orbit is completed with the source-detector pair traveling along the same circular path, but at different relative orientation. Each orbital scan is formed by a continuous succession of individual scan segments. The integral intensity of the beam passing through the subject during each scan segment is detected and recorded for computing the X-ray transmission or absorption characteristics of the subject. The data accumulated from the scan segments is processed by computational processing circuitry to produce a reconstructed image in the plane of interest. Such computational apparatus can also be used to operate the scanner in accordance with a predetermined operational sequence.
In a modification of the "orbital" system, multiple detectors, closely spaced about the circular path, have been used with a common X-ray source. Use of multiple detectors enables, in some circumstances, production of good image resolution after only a single orbit scan of the source and detectors about the subject, rather than requiring several orbits. In effect, the single detector multiple scan approach is traded off, at least in some circumstances, for a multiple detector, single scan approach. This latter approach is described in the DUAL MOTION, the SINGLE MOTION, and the BACKSIDE SCANNING applications, referenced above.
Reconstruction transaxial tomography systems of both described types have commonly utilized a computational technique known as "back projection" for processing the collected radiation intensity data, image. The detected intensity of the X-ray beam passing through the subject along a given narrow path (defined by a single scan segment) is "back projected", or attributed, to all points in a reconstruction matrix which corresponds to the path of the beam. The values of radiation transmission intensity measured for all such paths during all the scans are back projected in the matrix to produce a segment-by-segment and scan-by-scan information buildup, or reconstruction, of the image. that
More specifically, each value of the radiation transmission as it is back projected in the matrix for a given path is kept constant, and the respective values of each back projection at points of intersection of the respective paths are combined. Each point on the reconstructed image is therefore representative of the sum of the back projected intensities of the paths passing through the point. This technique is described in Kuhl, "A Clinical Radioisotope Scanner for Cylindrical and Section Scanning," PROC. SYMP., Athens 1964, Medical Radioisotope Scanning, I.A.E.A., Vienna, 1, 273, 1964, hereby expressly incorporated by reference.
The back projection technique has been improved with the introduction of filtered back projections and data processing using Fourier analysis. A discussion of Fourier reconstruction using filtered back projections is set forth in Chesler, "Positron Tomography and Three Dimensional Technique," PROC. SYMP. on Radio-nuclei Tomography, New York, N.Y., 1974. An algorithm for processing the data using convolutions on a digital computer is given in Shepp, et al, "Some Insights into the Fourier Reconstruction of a Head Section," Bell Laboratories, Murray Hill, N.J., 1974. Both the publications referred to in this paragraph are hereby expressly incorporated by reference.
In the orbital type of reconstruction tomography, the intensity of the detected radiation has been measured by integrating the data received from the detectors over the discrete time intervals during which the detectors traverse the respective scan segments, to produce signals each representing the average detected intensity over a segment. Early proposals for doing this employed a conventional integrator for integrating the data signal through the integration interval and an analog-to-digital converter for converting the integrated data signal to a digital value. This digital value, when compared with the duration of the integration interval, corresponding to the magnitude of the scan segment, represented the average detected intensity of the beam over the associated segment.
The method of processing the signals from the radiation detector in the described orbital system above requires integration over selected scan segments which often vary from one operating sequence to another of rotation of the orbital movement. In an operating sequence requiring repeated orbital scans, the scan segments of one scan are respectively rotationally offset by a small distance from the corresponding scan segments of a previous orbital scan. The reasons for this requirement in the operating sequence are explained in detail in the specifications of the DUAL MOTION, SINGLE MOTION and BACKSIDE SCANNING patent applications, expressly incorporated here by reference.
It is necessary to provide apparatus to accurately delineate the scan segment boundaries, this delineating apparatus should indicate to the computational apparatus the traversal of each scan segment by the detector and source. Since changes are often required in the size orientation of path segments, apparatus indicating traversal of scan segments must be flexible enough, to accommodate such variations.
In a previous proposed reconstruction transaxial tomographic scanner of the orbital motion type it has been proposed to generate signals indicating the rotational orbital position of the source and detector by a digital shaft encoder. Such an encoder was to be connected to a rotatable shaft mechanically connected to rotate as a function of the orbital rotation of the source and detector. In response to the rotation of the shaft, the digital encoder was to generate electrical signals indicating, within a given tolerance value, the rotational position of the source-detector combination.
While this proposed might have been satisfactory for some applications of tomographic scanners, the exacting requirements of precision, and flexibility of operating sequences of such scanners, have evoked a desire for greater accuracy (lower error tolerance) in the means for indicating scan segment traversal by the source-detector. The minute size of scan segments often used in operation of these scanners, (which minute size enhances system resolution) the fraction of degree offset orientation between scan segments of different scan orbits, and the variability of these sizes and offsets, has heightened the need for improved accuracy and resolution in providing indications of scan segment traversal.
Moreover, scanning orbit speeds vary by ratios as great as sixteen to one, (depending on the operational sequence selected) and the motion indicating control system must be capable of maintaining accuracy and stability throughout the full range of useful scan speeds.